1. Field of the Invention
The present invention relates to camera exposure time control circuitry and more particularly pertains to such control circuitry of the type in which a signal representative of scene brightness, measured through a camera objective, is stored in a storing device for controlling exposure time in accordance with the stored signal.
2. Description of the Prior Art
In a camera in which scene brightness is measured by a photocell through a camera objective (TTL light measuring type) and the photocell is blocked from scene light while the shutter is in operation, it is necessary to store the result of such light measurement carried out prior to the shutter operation, so that the shutter is controlled automatically in accordance with the stored signal. This TTL-light-measuring and signal-storing type exposure time control system is well known in the art, for example as shown in USP at Nos. 3,815,148, 3,733,984 and 3,977,011.
FIG. 1 schematically illustrates such an exposure control circuit of the TTL-light-measuring and measured-value-storing type. In the Figure, circuit A generates an output signal representative of an exposure time suitable for given exposure conditions, e.g. measured scene brightness, set diaphragm aperture value and set film sensitivity value, in accordance with a electrical calculation using signals corresponding to the set values and a signal commensurate with the scene brightness. The output signal of the circuit A is stored in a storing capacitor Cm in the form of voltage through a storing switch S3. Circuit B controls exposure time in accordance with the signal stored in the capacitor Cm in the manner that the circuit B deenergizes an electromagnet Mg to initiate shutter closing after a lapse of a time period dependent on the stored signal.
At an initial stage of the camera photographing operation, the storing switch S3 is closed, with a power switch S1 and an electromagnet energizing switch S2 being open. Upon depression of a shutter release button (not shown), the power switch S1 is closed to energize and actuate the circuit A and the exposure time representative signal from the circuit A is stored in the capacitor Cm through the closed switch S3. When the release button is depressed further, the switch S3 is opened to fix the signal in the capacitor Cm before a reflecting mirror is removed from its viewing position and a diaphragm is stopped down and switch S2 is closed to energize the electromagnet Mg through the circuit B, before shutter opening is initiated in response to the completion of mirror movement and stopping-down of the diaphragm. Then, time counting in the circuit B is commenced simultaneously with actuation of the shutter.
In this prior art circuitry, if the light measuring circuit A has a high output impedance, it takes considerable time for the capacitor Cm to be charged to the output level of the circuit A so that the voltage of the storing capacitor can not quickly follow changes in output of the circuit A, i.e. changes in scene brightness. In this case, the storing capacitor fails to store the scene-light-representative signal properly if the shutter release button is depressed down rapidly without a pause between closing of switch S1 and initiation of the operation of the mirror and diaphragm.